The identification and recognition of persons via biometric features is becoming increasingly important. Aside from other detectable biometric features, the fingerprint and handprint play a prominent role.
In systems for acquiring fingerprints and handprints, one must distinguish between two systems of this type which are used for a verification, i.e., a one-to-one comparison, for example for entry control and access control, and other systems which are used for personal identification, i.e., for recording and searching comparison data in a database administered by government authorities, for example, for border controls, aviation security and for police use.
For the latter systems, there is a great number of demands with respect to quality, resolution and fidelity to the original of the recorded images of skin textures. Further, there is a high degree of standardization based on the requirements catalog of the FBI, on the one hand, to ensure indisputable identification and, on the one hand, to allow comparison between datasets which were recorded by different systems. This requirements catalog includes six important parameters.
First, the systems must have a resolution of either at least 500 ppi (pixels per inch) or at least 1000 ppi. There may be no certification below these resolutions. One of the most important parameters for certification is the contrast transfer function (CTF). In this respect, the requirements catalog defines exactly the minimum value that the CTF must have in the corresponding spatial frequencies in the image. FIG. 7 shows these CTF standards of the FBI for a 500 ppi system as a function of spatial frequency (lp/mm) as a dashed curve (504)—at 10 lp/mm, the contrast must reach at least 25.8%. In case of a 1000 ppi system, a contrast of at least 28% must be achieved at 20 lp/mm A further important parameter is the signal-to-noise ratio (SNR) which must be at least 42 dB independent from the resolution. Further requirements include low distortion of <1%, the presence of at least 200 different grayscale values and a homogeneously illuminated image field both in the near pixel environment and in the image overall. The last parameter serves as a control for preventing the occurrence of image falsification. This means that unusual artefacts are explicitly looked for in the images to detect image manipulation.
All of the requirement criteria require a balanced and high-quality system design. In an optical system, this means, for example, that the requirements must be satisfied not only by the recording sensor, but also by the illumination and all other components necessary for imaging.
Currently, the optical arrangements chiefly used for acquiring fingerprints and handprints with the high quality requirements corresponding to the requirements catalog of the FBI, for example for forensic purposes or for personal identification at border controls, are based on the principle of frustrated total internal reflection. In this case, owing to mechanical and optical requirements, a prism is used in which the provided surface for recording the print must be larger than the required surface for recording the print. The resulting size of the prism, commonly the largest component in the recording channel, decisively influences the minimum constructional size and minimum mass of a device of this kind. On the other hand, the high image quality allows fast and reliable recognition and identification of persons, particularly for applications in which, along with forensic accuracy (FBI requirements), a high person throughput is expected, e.g., at border controls.
The disadvantages of these types of arrangements with prisms, apart from the size and mass, are the required complex mechanical components and a complicated assembly and adjustment.
Miniaturized arrangements with imaging optics, as in U.S. Pat. No. 7,379,570 B2, generally do not satisfy FBI requirements while still limiting minimization of the devices owing to the space required for the optical beam path. Ultrasonic sensors or piezo sensors as are known, e.g., from U.S. Pat. No. 4,394,773, and capacitive sensors as described, e.g., in U.S. Pat. No. 5,325,442, cannot optically capture fingerprints. Devices based on ultrasonic sensors are not yet commercially available. Up to the present day, existing capacitive sensors in turn are only for capturing one or two fingers. Membrane keyboards as described, for example, in US 2005/0229380 A1 do not satisfy the necessary FBI criteria.
Approaches have already been described in US 2012/0321149 A1 for combining the advantage of high image quality which can be achieved with frustrated total internal reflection with a small constructional size. In the fingerprint sensor disclosed therein, the finger is placed on a transparent substrate, the sensor being located directly under the latter. As is the case in arrangements with prisms, the brightness profile corresponding to the fingerprint comes about in that the skin ridges (further on: ridges) resting on the surface of the substrate frustrate the internal reflection of light from the light source, while there is no contact between skin valleys (further on: valleys) and surface of the substrate and the light from the light source is internally reflected there at the surface of the substrate. Accordingly, a negative image of the fingerprint is formed on the light-sensitive areas of the sensor array. Thus this solution assumes that the upper substrate has a minimum thickness so that the light generates an image of the fingerprint through reflection at the placement surface on the light-sensitive elements of the sensor array. Further, it is necessary that the illumination meets certain requirements with respect to incident direction and aperture angle or collimation, which appreciably increases the technical expenditure on illumination as well as the space requirement. The constructions for illumination presented cannot be implemented for large recording surfaces for more than one or two fingers or, if so, only at considerable expense.
A further concept for a flat construction without imaging optics is described in U.S. Pat. No. 7,366,331 B2. In this case, light is coupled laterally into the finger by means of areal illumination and is guided from the latter to the deposited parts of the skin. But this light guidance functions exclusively for light components in the red spectral range and infrared spectral range. Shorter-wavelength light, starting from the green spectral region, is absorbed by the blood in the finger. If the finger contacts the transparent layer between finger and sensor, light from the ridges preferably couples into the transparent layer and can accordingly be detected by the surface sensor. Thus this concept stipulates illumination wavelengths within the transparency range of the finger (NIR range and IR range) and entails substantial problems with respect to ambient light.
The use of IR filters and IR illumination proposed in U.S. Pat. No. 7,366,331 B2 mitigates these problems but only at the cost of reduced sensitivity of the utilized sensors and higher absorption of the finger, which worsens the signal-to-noise ratio. Utilized narrowband spectral filers must be adapted very precisely to the wavelength of the illumination and generate additional expense. Further, the lateral illumination causes problems with the homogeneity of the lighting; in particular, it prevents the simultaneous capture of more than one finger because the fingers would shadow one another. Therefore, this concept is only suitable for capturing an individual finger. Further, a light shield is needed in order to prevent portions of the illumination from reaching the sensor directly. The lateral illumination and light shield increase the size of the device and make it costlier, less flexible and more susceptible to malfunction.